My experience is the same as that of @TonyBarrett.
I, too would like to see this, but it is not straightforward. In layer B (which I use for lighttable), for example, I have the last 4 buttons on the bottom row assigned to 4 different export presets. So the moment I press one, one of the other three (if illuminated) would need to be extinguished.
A simple on/off toggle (like enabling a module - e.g. profiled denoise) may possibly be easier.
I’d also like to be able to trigger an event on changing from layer A to layer B and vice versa, but that is something internal to the X-touch and does not generate any MIDI messages.
I have also wished for the ability to imitate the mouse wheel with the rotary knobs.
It is actually pretty freaking awesome how well the X-touch can work with darktable, given it was designed with a very different intended use. So kudos to the developers for the redesign of the shortcut system to allow easy programming of midi-controllers in general and the X-touch mini in particular.